Differentially Accelerated Electron and Hole Diffusion in MAPbI₃ Film Surface Treated by a Quaternary Ammonium Halide for High‐Efficiency Solar Cells

Abstract

Surface treatment with moisture‐tolerant molecules is an effective approach for improving the long‐term stability of perovskite solar cells (PSCs), but may bring drawbacks to carrier dynamics and power conversion efficiency (PCE). Although PSCs with both high stability and PCE have been realized by specially designed molecules recently, the underlying mechanism boosting the superior PCE is still not clear, limiting the further optimization of the photovoltaic performances. Herein, by monitoring carrier dynamics in tetra‐ethyl ammonium iodine (TEAI)‐treated MAPbI3 using femtosecond transient reflectivity technique with the rationally selected pump‐probe wavelengths, the electron and hole diffusivities are found being 7 and 3 times faster than those in the pristine MAPbI3, respectively. The differentially enhanced electron and hole diffusivity and the associated charge separation might originate from TEAI, which can effectively passivate both the negatively and positively charged defects. This study indicates that quaternary ammonium halide might be an efficient tool for improving both moisture stability and PCE of perovskites due to its hydrophobic organic cations ready to anchor along the surface and anions with a deeper penetration.

Post-Treatment-Free Dual-Interface Passivation via Facile 1D/3D Perovskite Heterojunction Construction

“…Meanwhile, a faster response on the photocurrent decay (Figure S12) is exhibited by the 1D/3D PSCs (5.16 μs) than the FAPbI 3 device (8.63 μs), illustrating slightly faster charge transport or collection and fewer trapping effects with the buildup of the dual-heterojunction structure. This observation on carrier dynamics corroborates with some previous studies on the positive influence of the 1D TEAPbI 3 ultrathin layer when capped on a 3D perovskite film. , The recombination state in PSCs can also be demonstrated by the ideality factor (n id ), which is extracted from analyzing the relationship between the open-circuit voltage ( V oc ) and light intensity . As shown in Figure d, the n id value decreased from 1.84 to 1.38 for 1D/3D PSCs, suggesting that the charge-carrier recombination has been effectively suppressed.…”

“…This diffusion preference of TEA + toward the interface can be attributed to its structural and chemical properties. 30 The slightly larger size and hydrophobicity of this quaternary alkylammonium cation deem its proximity to the interface instead of residing inside the 3D bulk structure, 35 which resembles the function of a surfactant in nanoparticle synthesis. 36 In detail, during the deposition process of the FAPbI 3 film, the TEA + cation and the TFA – anion on the SnO 2 substrate would interact with the components in the perovskite precursor solution.…”

“…This observation on carrier dynamics corroborates with some previous studies on the positive influence of the 1D TEAPbI 3 ultrathin layer when capped on a 3D perovskite film. 30 , 35 The recombination state in PSCs can also be demonstrated by the ideality factor (n id ), which is extracted from analyzing the relationship between the open-circuit voltage ( V oc ) and light intensity. 40 As shown in Figure 3 d, the n id value decreased from 1.84 to 1.38 for 1D/3D PSCs, suggesting that the charge-carrier recombination has been effectively suppressed.…”